The preferred embodiment concerns a method, a computer program, a computer and a printing system for trapping of image data.
Color documents or document parts (such as, for example, images, color graphics or the like) are for the most part described by image data that are subdivided into color separations. This type of data subdivision in turn corresponds to many print output methods or apparatuses that print the image data in color separations on a recording medium, for example in the colors yellow (Y), magenta (M), cyan (C) and black (K) or in black and one or more of what are known as highlight color colors or the Océ Customer Toner® colors.
OCE Printing Systems GmbH develops and distributes corresponding digital electrographic printing systems. They are, for example, described in the publication “The World of Printers, Technologies of Océ Printing Systems”, Dr. Gerd Goldmann (Ed.), Océ Printing Systems GmbH, Poing, 7th edition (2002). Various offset and digital printing technologies are described on pages 249-286, various digital color printing systems are described on pages 287-325 and foundations of color printing are described on pages 233-248. Foundations of digital image processing are described on pages 209-232. Principles of highlight color printing are described on pages 246-248.
A digital printing system for double-sided monochrome and/or color printing of a recording medium is known from WO 98/39691 A1. A method for preparation of an image point file is known from the international patent application Nr. PCT/EP2004/00700 (publication number WO 2005/001765 A2), in which contiguous areas of the image are determined from the image points.
Methods for trapping image data are known from U.S. Pat. No. 5,581,667, EP-A2-484 890, US 2003/0090689 A1, U.S. Pat. No. 4,931,861, EP-A2-929 189, DE-A1-199 12 511, US 2001/0055130 A1 and EP-A2-833 216.
The aforementioned publications are herewith incorporated by reference into the present specification.
What is known as the registration problem is present both in digital printing and in offset printing. It thereby deals with the circumstance that, given a plurality of printing processes and given one sheet of paper, due to mechanical tolerances it cannot be guaranteed that the positioning of the paper is always exactly the same in all printing processes. The problem occurs in single-color printing when the front side and the rear side are printed separately or given multi-color printing on one side.
In front side and rear side printing this problem interferes when, for example, one frame per front side and rear side is printed and these frames do not lie exactly atop one another, which one notices when the page is held up to the light.
Given multicolor printing the colors are offset relative to one another. As long as the different colors do not touch, this does not stand out any further. If the colors touch, due to the offset the colors are printed over one another at the contact line, which leads to an adulteration of the color impression, or a white gap (flash) remains at the contact line.
While the adulteration of the color impression is for the most part still tolerable, the flashes are extremely conspicuous, as is shown by the comparison of exactly positioned colors in FIG. 1A and colors positioned offset in FIG. 1B.
To remedy the flashes problem it is known to enlarge or spatially over-fill the lighter colors. Although a greater superimposition of the colors is therewith obtained, the flashes disappear, as is shown by the comparison of colors positioned offset in FIG. 2A and superimposed colors in FIG. 2B. Given the enlargement of an object, in the later printing process care must be taken that the superimposing portion is printed transparent since otherwise the problem shifts to the edge of the enlarged object.
The method just described, which method remedies this problem, has the name “trapping” (overfilling). Trapping is offered in different products on the market. For example, it is a component of raster image processors (RIPs) of the page description language (PDL) Adobe PostScript® Level 3, the software SuperTrap® offered by the company Heidelberger Druckmaschinen AG or the software TrapWise® that is offered by the company Creo.
Trapping can be implemented in two different manners. Trapping can be dealt with on the object level or on the bitmap level.